When an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the storage panel; sequentially scanning the panel with a stimulating light such as a laser beam to emit a stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then kept for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and an energy-storing phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support can be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein, or otherwise can comprise agglomerate of an energy-storing phosphor without binder. The binder-less phosphor layer can be formed by a gas phase-accumulation method or by a firing method. For example, in the gas phase-accumulation method, the phosphor or material thereof is vaporized (or sputtered) and accumulated on a substrate to prepare a layer of the phosphor in the form of columnar crystals. The prepared phosphor layer consists of only the phosphor, and there are gaps among the columnar crystals of phosphor. Accordingly, the stimulating light can be applied efficiently enough and the emission can be collected also efficiently enough to improve the sensitivity. In addition, since the stimulating light is kept from scattering horizontally, an image of high sharpness can be obtained.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. It is, however, still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of as high quality (in regard to sharpness and graininess) as possible.
As a method for providing the protective layer, the wet coating method is known. It is also known to glue a protective film onto the phosphor layer with adhesive, namely, via an adhesive layer.
For example, UP-B-63-25320 discloses an adhesive layer satisfying the condition of:dav.×μ<λ/2 or dav.×μ>λin which dav. is the average thickness of the adhesive layer, μ is the refractive index of the adhesive layer, and λ is the wavelength of the stimulating light. Disclosed herein is an adhesive layer formed on a phosphor layer comprising a binder and stimulable phosphor particles dispersed therein.
JP-A-2001-141896 discloses a radiation image storage panel comprising a stimulable phosphor layer and a protective film laminated thereon. The protective film has a thickness of 1 to 10 μm and is made of fluorocarbon resin. The phosphor layer described in the publication comprises a binder and a stimulable phosphor. In an example of the publication, an adhesive layer of 2 g/m2 is formed by wet coating.
JP-A-2004-37448 describes a stimulable phosphor screen having a phosphor layer formed by vapor-deposition. The phosphor layer comprises phosphor in the form of needle-like crystals, and gaps among the crystals are partly filled with a polymer compound to improve the mechanical strength of the phosphor layer. According to the publication, known binder polymers can be used as the polymer compound. The publication also describes that a coating solution for forming a protective layer can be spread on the phosphor layer, so as to form the protective layer and, at the same time, to fill the gaps in. It is further described that the gaps are filled with the polymer compound in a depth of at least 5 μm.
The applicant has already proposed a radiation image storage panel comprising a stimulable phosphor layer, an adhesive layer and a transparent moisture-proof film (U.S. Ser. No. 11/008,225). The transparent moisture-proof film comprises a transparent thin film having a thickness of 1 to 10 μm and a moisture-proof layer formed thereon, and is glued on the phosphor layer via the adhesive layer having a thickness of 0.1 to 10 μm (preferably 0.1 to 3 μm). As a result of further study, the applicant has found that this storage panel often gives a blurry radiation image if the phosphor layer has a structure without binder (namely, binder-less structure), particularly, if the phosphor layer consists of phosphor in the form of columnar crystals (among which there are gaps). The applicant's study has further revealed that, when the adhesive layer is formed on the phosphor layer to glue the protective layer or the like, material of the adhesive layer partly intrudes into the gaps to make the radiation image blurry. Even if the adhesive layer intrudes in a depth of no more than several micrometers, the resultant radiation image is remarkably impaired in sharpness.